Internal combustion engines such as gasoline engines exhaust a complex mixture of air pollutants. The air pollutants are composed of solid particulate matter and gaseous compounds including nitrous oxides (NOx). Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of solid particulate matter and gaseous compounds emitted to the atmosphere from an engine is regulated depending on the type of engine, size of engine, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions has been to implement exhaust gas recirculation (EGR). EGR systems recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas, which is redirected to a cylinder of the engine, reduces the concentration of oxygen therein, thereby lowering the maximum combustion temperature within the cylinder. The lowered maximum combustion temperature slows the chemical reaction of the combustion process, thereby decreasing the formation of nitrous oxides. In addition, the particulate matter entrained in the exhaust is burned upon reintroduction into the engine cylinder to further reduce the exhaust gas by-products. One such EGR system is disclosed in U.S. Pat. No. 6,237,336 (the '336 patent), issued to Feucht et al. on May 29, 2001.
The '336 patent discloses a spark-ignited gasoline engine having a plurality of pistons reciprocatingly disposed within combustion chambers of the engine. Exhaust gases, which are discharged from the combustion chambers, flow through an exhaust manifold to a turbine. The turbine drives a compressor to force a portion of the exhaust gases and air through an air-to-air aftercooler (ATAAC) back into the combustion chambers for subsequent combustion.
Although the EGR system of the '336 patent may reduce the amount of NOx and particulate matter exhausted to the atmosphere, it may be limited. In particular, as the amount of exhaust gas recirculated back into the combustion chambers of the engine increases, the air-to-fuel ratio of the engine likewise increases. Eventually, the air-to-fuel ratio will reach a lean burn condition that can no longer can support normal spark ignition. When spark ignition of the air/exhaust gas/fuel mixture fails, operation of the engine can become unstable and/or unpredictable. In addition, if the amount of exhaust gas recirculated back into the engine is limited to a predetermined air-to-fuel ratio that provides for spark ignition, the amount of NOx and particulate matter removed from the exhaust flow may be insufficient to comply with emission regulations.
The disclosed EGR system is directed to overcoming one or more of the problems set forth above.